Smart glasses are a special form of a head-mounted display. A common form of head-mounted display uses screens which are worn in front of the eyes and present the user with computer-generated images or images taken by cameras. Such head-mounted displays are frequently bulky and do not allow for a direct perception of the surroundings. Only recently have head-mounted displays been developed that are capable of presenting to the user an image taken with a camera or a computer-generated image without preventing the immediate perception of the surroundings. Such head-mounted displays, hereinafter called smart glasses, allow for the use of this technology in everyday life.
Smart glasses can be provided in different ways. One type of smart glasses which is characterized particularly by its compactness and esthetic acceptance is based on the principle of wave guidance in the eyeglass lens. Light generated by an image generator is collimated outside of the eyeglass lens and coupled in via the end face of the eyeglass lens, from where it spreads by means of a plurality of total reflection up to the front of the eye. An optical element located there subsequently decouples the light in the direction of the eye pupil. The coupling into the eyeglass lens and the decoupling from the eyeglass lens can be either diffractive, reflective, or refractive.
For diffractive coupling and decoupling, diffraction gratings with almost identical line count are used as coupling and decoupling elements, wherein the greatly dispersive effects of the individual gratings are compensated among each other. Decoupling elements based on diffraction gratings are described, e.g. in US 2006/0126181 A1 and in US 2010/0220295 A1. Examples for smart glasses with reflective or refractive coupling and decoupling elements are described in US 2012/0002294 A1.
Smart glasses, in which an imaging beam is guided a number of (total) reflections from a coupling element to a decoupling element, have, regardless of whether diffractive, reflective, or refractive elements are used as coupling and decoupling element, the problem of the so-called “footprint overlap” in common. This problem, which limits the size of the field of view (FOV) as well as the size of the exit pupil of the smart glasses at the location of the eye box, and as a result requires a relatively great eyeglass thickness, shall be explained in the following in more detail using FIGS. 1 and 2.
The eye box is the three-dimensional area of the light tube in the imaging beam path, in which the eye pupil can move without resulting in a vignetting of the image. Since in smart glasses, the distance of the eye relative to the smart glasses is essentially constant, the eye box can be reduced to a two-dimensional eye box which only takes into account the rotational movement of the eye. In such case, the eye box corresponds essentially to the exit pupil of the smart glasses at the location of the entrance pupil of the eye. As a rule, the latter is constituted by the pupil of the eye. Even though smart glasses are a system, in which an imaging beam path runs from the image generator to the exit pupil of the smart glasses, it is useful for understanding the “footprint overlap” to look at the beam path from the reverse direction, i.e. from the exit pupil to the image generator. Therefore, in the following explanations, a light tube proceeding from the exit pupil of the smart glasses shall be examined, wherein the boundaries of the light tube are determined by the field-of-view angle of the beams expanding from each point of the eye box in the direction of the eyeglass lens. After diffraction on the inner surface 3 of the eyeglass lens 1, the beams impinge in the light tube on the outer surface 5 of the eyeglass lens 1. In it, the decoupling structure 7, which extends in horizontal direction from point B to point C, is located. The distance between points B and C is determined by the desired expansion of the light tube which, in turn, depends on the desired size of the eye box 9 and the desired field-of-view angle. Here, the field-of-view angle is primarily the horizontal field-of-view angle which, relative to the visual axis, relates to the angle at which the horizontal boundary points of the image field impinge in the pupil. In this case, the visual axis denotes a straight line between the fovea of the eye (point of sharpest vision of the retina) and the center of the image field. FIG. 1 shows the profile of the light tube at an eye-box diameter E and a thickness d of the eyeglass lens 1 for a relatively small field-of-view angle. All beams of the light tube are deflected or reflected from the decoupling structure 7 in the direction of the inner surface 3 of the eyeglass lens 1 and from there by means of total reflection reflected back to the outer surface 5 of the eyeglass lens 1, from where they are reflected back again under total reflection to the inner surface 3 of the eyeglass lens 1. This back-and-forth reflection occurs until the coupling element is reached, from where the light tube then continues to extend in the direction of the image generator.
If, as shown in FIG. 1, the field-of-view angle is relatively small, the beams of the light tube, after the first total reflection on the inner surface 3 of the eyeglass lens 1, impinge on an area of the outer surface 5 of the eyeglass lens 1 which lies outside of the decoupling element 7 (in FIG. 1 on the right next to point B). However, if a large field-of-view angle is desired, as is shown in FIG. 2, a correspondingly enlarged decoupling structure 7′ is required. However, this causes beams of the light tube, which impinge on the section of the decoupling structure 7′ located between points A and C, to be reflected back after the first total reflection on the inner surface 3 of the eyeglass lens 1 to an area of the outer surface 5 of the eyeglass lens 1, in which the decoupling structure 7′ is still located. This area, hereinafter called overlap area, is located in FIG. 2 between points B and D. Due to the presence of the decoupling element, which can be a diffractive or reflective decoupling element in the depiction selected in FIG. 2, the beams reflected from the inner surface 3 of the eyeglass lens 1 to the area between B and D are not reflected back in the direction of the inner surface 3, and so they are lost for imaging.
A similar problem also arises if the diameter of the eye box is enlarged instead of the field-of-view angle. In this case, there would also be points A and C, between which an area is located that reflects beams in the direction of the inner surface 3 of the eyeglass lens 1, which, under total reflection, are reflected back to an area of the decoupling structure 7′ denoted by points B and D and are thus useless for imaging. The same would also apply if the eye-box diameter E and the field-of-view angle were maintained and the thickness d of the eyeglass lens were decreased. In other words, a sufficiently large eye-box diameter E at a sufficiently large field-of-view angle can only be achieved with a specific minimum thickness d of the eyeglass lens.
At this point, it must be noted once again that for the above description, the beam path was reversed, and that the actual beam path extends from the image generator to the exit pupil of the smart glasses. However, this changes nothing on the basic observation, since beams coming from the image generator, which impinge on the decoupling structure 7′ in the area between points B and D, are not reflected into the exit pupil because they are not reflected back by means of total reflection in the direction of the inner surface of the eyeglass lens, which, however, would be necessary in order to reach the area of the decoupling structure 7′ between points A and C, from where it would be possible to decouple them in the direction of the exit pupil.